The present invention relates generally to improved adhesive compositions. More particularly, the present invention relates to improved pressure sensitive adhesive compositions containing graft copolymers.
Adhesives have been used since antiquity to hold substrates together via surface attachment. The term xe2x80x9cadhesivexe2x80x9d, as used herein, is a substance that is typically a liquid or tacky semisolid, or at least for an instant to contact and wet a surface, and be applied in a relatively thin layer to form a useful joint capable of transmitting stresses from one substrate to another. The term xe2x80x9cpressure sensitivexe2x80x9d, as used herein, refers to adhesives which typically do not undergo hardening after they have been applied to the surface of the substrate and the joint is formed. These adhesives are capable of holding substrates together when the surfaces are mated under briefly applied pressure at room temperature.
The properties of tack, peel strength and shear resistance, which are frequently mutually exclusive properties, may be highly important in tailoring an adhesive composition that is suitable for a particular application. Tack is a measure of viscous flow under conditions of fast strain rates and low stress magnitudes and generally relates to the initial attraction of an adhesive to a substrate. Peel strength is a measure of resistance to flow at intermediate strain rates and moderate to high stress magnitudes and generally relates to the measure of bond strength between an adhesive and a substrate. Shear resistance is a measure of resistance to flow at intermediate stress magnitudes and generally relates to the internal or cohesive strength of an adhesive.
Pressure sensitive adhesives (xe2x80x9cPSAsxe2x80x9d) may be generally comprised of rubber, acrylic or silicone based formulations and may be manufactured via such methods as solvent, emulsion, or hot melt processes. Pressure-sensitive adhesive compositions based upon aqueous emulsions and dispersions of acrylic are known and widely used. Exemplary of such pressure sensitive adhesives include, for example, graft copolymers. The term xe2x80x9cgraft copolymersxe2x80x9d, as used herein, refers to macromolecules formed when polymer or copolymer chains are chemically attached as side chains to a polymeric backbone. Generally, the side chains are of a different polymeric composition than the backbone chain. Because graft copolymers often chemically combine unlike polymeric segments in one molecule, these copolymers have unique properties compared to the corresponding random analogues. These properties include, for example, mechanical film properties resulting from thermodynamically driven microphase separation of the polymer.
The term xe2x80x9ccomb copolymerxe2x80x9d, as used herein, refers to a type of graft copolymer, where the polymeric backbone of the graft copolymer is linear, and each side chain of the graft copolymer is formed by a xe2x80x9cmacromonomerxe2x80x9d that is grafted to the polymer backbone. The term xe2x80x9cmacromonomersxe2x80x9d, as used herein, are low molecular weight polymers having at least one functional group at the end of the polymer chain that can further polymerize with others monomers to yield comb copolymers. See e.g., Kawakami in the xe2x80x9cEncyclopedia of Polymer Science and Engineeringxe2x80x9d, Vol. 9, pp. 195-204, John Wiley and Sons, New York, 1987. The term xe2x80x9clinearxe2x80x9d, as used herein, is meant to include polymers where minor amounts of branching has occurred through hydrogen abstraction that is normally observed in free radical polymerizations. The comb copolymers are commonly prepared by the free radical copolymerization of macromonomer with conventional monomer (e.g., ethylenically unsaturated monomers).
Comb copolymers prepared with water-insoluble macromonomers have been predominantly prepared using bulk and solution polymerization techniques. However, such processes typically involve the use of solvent or monomer as the medium in which the polymerization is conducted. The use of such materials is undesirable, for example, due to toxicity concerns. Thus, efforts recently have focused on developing methods for preparing comb copolymers that may be suitable for use as PSAs via an aqueous emulsion process.
A parameter which is frequently relevant for selecting a grafted copolymer or comb copolymer that imparts the proper balance of properties for the adhesive composition, particularly for PSA compositions, is the respective glass transition values (Tg) of the xe2x80x9chard phasexe2x80x9d and the xe2x80x9csoft phasexe2x80x9d. As used herein, the term xe2x80x9chard phasexe2x80x9d generally refers to the polymer or copolymer side chains or grafts, whereas the term xe2x80x9csoft phasexe2x80x9d generally refers to the polymeric backbone of the grafted copolymer. It is generally believed that the Tg of the hard phase strongly influences the shear properties of the resulting adhesive compositions.
The degree of compatibility for the hard and soft phases also effects the shear and other properties of the adhesive compositions to a lesser degree. It is important that the copolymer side chains, or grafts, have low or no compatibility with the polymeric backbone so that separate phases are formed.
Compatibility, as used herein, refers to a measure of the mutual solubility of two materials, such as the hard and soft phases of the graft or comb copolymer. Compatible blends may be characterized by (1) the existence of a single homogeneous phase which contains no discrete domains of either component, and (2) a single glass transition temperature for the blend of components as discussed in P. B. Rim and E. B. Orler, xe2x80x9cDependence of Tg on Composition for a Compatible Polymer/Oligomer Blendxe2x80x9d, Macromolecules, Vol. 20, pp. 433-435 (1987).
In blends of polymers or blends of additives with polymers, a further aspect of compatibility relates to the differences in refractive indices between components. A lack of compatibility is generally evidenced by haziness in the dried adhesive film due to large domains of individual components of differing refractive index. Compatibility is typically favored between materials which are similar in chemical and/or physical characteristics. To effectively modify the performance of an adhesive containing graft or comb copolymers, the selected additives should preferably be at least partially compatible with the soft phase of the copolymer and have very limited, or no compatibility, with the hard phase.
U.S. Pat. No. 4,554,324 to Husman et al. (xe2x80x9cHusmanxe2x80x9d) discloses PSA compositions that comprise a polymerized acrylic or methacrylic acid ester backbone having grafted pendant polymeric moieties. The grafted pendant polymeric moieties are comprised of macromonomers that may be prepared by anionic or free-radical polymerization processes using alkali metal hydrocarbons, alkoxide salts, or free-radical initiators, respectively. The reactive double bond of the macromonomer is an acrylate or methacrylate linkage to a desired polymeric repeat unit such as styrene or methyl methacrylate.
Husman teaches the use of macromonomers with acrylic compositions to reinforce the cohesive strength, or shear resistance, of polymers as seen in shear strength while maintaining a desirable balance of other PSA properties such as peel and tack. The adhesive compositions in Husman are made into films from solvent solutions or via melt related coating processes, such as extrusion or hot melt coating. Further, Husman does not teach the use of polymeric additives such as tackifiers to improve the properties of the PSA compositions.
U.S. Pat. No. 5,006,582 to Mancinelli (xe2x80x9cMancinellixe2x80x9d) discloses acrylic hot melt PSA compositions that contain acrylic comb copolymers. Mancinelli teaches that the acrylic comb copolymers, which generally consist of a methyl methacrylate macromonomer repeat unit that is linked to an acrylate or methacrylate terminal double bond, are made via group transfer polymerization. Mancinelli discloses the use of cobalt chain transfer agents to produce macromonomers with a nonacrylate type of terminal double bond that still reacts well with acrylates and methacrylates.
Mancinelli further teaches that the PSA properties and melt processability of these MMA graft copolymers can be greatly improved by adding certain classes of tackifying resins that maintain the water whiteness of the all-acrylic adhesive backbone. These types of tackifiers have improved stability to oxidation and light. The tackifiers disclosed in Mancinelli are completely hydrogenated polyaromatic copolymers blended with low levels of partially hydrogenated rosin esters.
U.S. Pat. No. 4,551,388 to Schlademan (xe2x80x9cSchlademanxe2x80x9d) discloses acrylic hot melt PSA compositions that are prepared by copolymerizing a vinyl aromatic monomer macromolecular monomer with alkyl acrylate esters, or optionally, mixtures of alkyl acrylate esters and acrylic acids or acrylamides. The polymerization is carried out in an organic solvent using a free radical initiator. After polymerization is completed, the solvent is removed to yield a xe2x80x9ctackyxe2x80x9d acrylate copolymer. Schlademan does not teach the use of polymeric additives such as tackifiers to improve the properties of the PSA compositions.
U.S. Pat. No. 5,578,683 to Koch et al. (xe2x80x9cKochxe2x80x9d) discloses PSA compositions that contain crosslinkable grafts of a high Tg macromonomer to an acrylic polymer backbone.
Other references, such as Shell Chemical Company product literature SC1757-93R (xe2x80x9cShellxe2x80x9d), which provides an overview of KRATON(trademark) Polymers, disclose that in order to process graft or block copolymers, the reinforcing, high Tg, phase separated domains must be dispersed by heating the copolymer above the Tg of the hard phase and applying shear as in the extrusion processes. The term xe2x80x9cblock copolymerxe2x80x9d, as used herein, refers to linear macromonomers formed by attachment of different polymers or copolymers at its ends. The phase separation structure reforms on cooling.
An alternate approach disclosed in Shell is to provide processing to dissolve polymers in solvents that are capable of dissolving both the hard and soft phases of the block or graft copolymers to provide a polymer solution. These polymer solutions can then be applied by conventional coating techniques. When the solvent evaporates, the phase separation reforms and the unique properties of the copolymers are once again obtained.
Shell also discloses that in room temperature applications where flammable and/or volatile solvents are undesired, block copolymers can be dispersed into water through various processes with suitable surfactants, or other means, to form emulsions. However, coatings formed from these emulsions are discrete or agglomerated particles rather than coherent films since the physically crosslinked structure of block copolymers generally does not allow coalescence at drying temperatures below the Tg of the hard phase. To remedy this, the end-user can add solvent that is capable of lowering the Tg of the hard phase to these emulsions to cause coalescence at ambient temperature. However, this remedy is undesirable for many applications because it may reintroduce volatile organic compounds.
The present invention seeks to provide improved adhesive compositions containing graft copolymers for use, for example, as pressure sensitive adhesives, that do not require the need to process the compositions as a melt or extrusion, or with a solvent to achieve proper film formation and other PSA properties. Instead, the adhesive compositions comprise water-insoluble graft copolymer that are dispersed within an aqueous medium. The adhesive compositions of the present invention may be coated onto substrates at temperatures well below the Tg of high Tg grafts without requiring the need for melt processing. These improvements in adhesive properties may be achieved without the use of solvents in the formulation of these acrylic graft copolymer emulsion adhesives. Moreover, the adhesive compositions of the present invention may desirably exhibit enhanced peel strength and tack while maintaining excellent shear resistance at elevated operating temperatures.
The present invention is directed, in part, to improved adhesive compositions. Specifically, in one embodiment, there are provided adhesive compositions that comprise from 30 weight percent to 70 weight percent of solids that are dispersed within an aqueous medium. The solids are comprised of water insoluble graft copolymers. The copolymers, in turn, comprise from 1 weight percent to 30 weight percent water insoluble macromonomer, and from 70 weight percent to 99 weight percent of polymerized units of at least one second ethylenically unsaturated monomer, based on the total weight of the copolymer. The macromonomer used to form the graft copolymer composition has a number average molecular weight (xe2x80x9cMnxe2x80x9d) of from 2,000 g/mole to 50,000 g/m and comprises from 85 to 100 weight percent of at least one first ethylenically unsaturated polymerized monomer, 5 mole percent or less of polymerized mercapto-olefin compounds, and 10 weight percent or less polymerized acid-containing monomer. In certain embodiments, the percentage of grafting of the macromonomer to the monomer in the graft copolymer particles ranges from 50% to 100%.
In a preferred embodiment, the adhesive composition further comprises from 0.1 to 60 weight percent, based upon dry weight of the solids of the copolymer, of an additive. The additive may be at least one additive selected from the group consisting of emulsifiers, defoamers, tackifiers, pigments, fillers, curing agents, thickeners, wetting agents, biocides, adhesion promoters, humectants, colorants, waxes, UV stabilizers, and antioxidants.
These and other aspects of the invention will become more apparent from the following detailed description.
The present invention is directed to improved adhesive compositions, particularly PSA adhesive compositions, comprising graft copolymers. The present adhesive compositions may desirably exhibit an improved balance of properties in comparison to adhesive compositions of the prior art. In particular, the adhesive compositions of this invention may exhibit an improved balance of tack, peel strength and shear resistance, preferably without the problems associated with melt or solvent processing.
The adhesive compositions of the present invention are comprised of grafted copolymers dispersed in an aqueous medium. The grafted copolymers, which are preferably in the form of solid particles, are preferably prepared by a method that includes (a) forming a macromonomer aqueous emulsion containing one or more water-insoluble particles of macromonomer; (b) forming a monomer composition containing ethylenically unsaturated monomer; and (c) combining at least a portion of the macromonomer aqueous emulsion and at least a portion of the monomer composition to form a polymerization reaction mixture. The macromonomer and ethylenically unsaturated monomer are then polymerized in the presence of an initiator to form the graft copolymer.
The macromonomer, present in the macromonomer aqueous emulsion as water insoluble particles, may be any low molecular weight water-insoluble polymer or copolymer having at least one terminal ethylenically unsaturated group that is capable of being polymerized in a free radical polymerization process. By xe2x80x9cwater-insolublexe2x80x9d it is meant having a water solubility of no greater than 150 millimoles/liter at 25xc2x0 C. to 50xc2x0 C. By xe2x80x9clow molecular weightxe2x80x9d, it is meant that the macromonomer has a Mn of from 2,000 to 50,000 g/mole. Preferably, the macromonomer has a Mn of from 2,000 to 50,000 g/mole, more preferably from 4,000 to 35,000 g/mole.
The macromonomer contains, as polymerized units, at least one type of ethylenically unsaturated monomer. Preferably, the ethylenically unsaturated monomer is selected such that the macromonomer is water insoluble, i.e., the macromonomer has low or no water solubility, as previously described herein. In preferred embodiments, the macromonomer is comprised of from 50 weight percent to 100 weight percent, more preferably from 85 weight percent to 100 weight percent, and even more preferably from 90 weight percent to 100 weight percent, of at least one ethylenically unsaturated monomer.
Suitable ethylenically unsaturated monomers for use in preparing macromonomer include, for example, methacrylate esters, such as C1 to C18 normal or branched alkyl esters of methacrylic acid, including methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, lauryl methacrylate, stearyl methacrylate; acrylate esters, such as C1 to C18 normal or branched alkyl esters of acrylic acid, including methyl acrylate, ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate; styrene; substituted styrenes, such as methyl styrene, xcex1-methyl styrene or t-butyl styrene; olefinically unsaturated nitriles, such as acrylonitrile or methacrylonitrile; olefinically unsaturated halides, such as vinyl chloride, vinylidene chloride or vinyl fluoride; vinyl esters of organic acids, such as vinyl acetate; N-vinyl compounds such as N-vinyl pyrrolidone; acrylamide; methacrylamide; substituted acrylamides; substituted methacrylamides; hydroxyalkylmethacrylates such as hydroxyethylmethacrylate; hydroxyalkylacrylates; basic substituted (meth)acrylates and (meth)acrylamides, such as amine-substituted methacrylates including dimethylaminoethyl methacrylate, tertiary-butylaminoethyl methacrylate and dimethylaminopropyl methacrylamide and the likes; dienes such as 1,3-butadiene and isoprene; vinyl ethers; or combinations thereof. The term xe2x80x9c(meth)xe2x80x9d as used herein means that the xe2x80x9cmethxe2x80x9d is optionally present. For example, xe2x80x9c(meth)acrylatexe2x80x9d means methacrylate or acrylate.
The ethylenically unsaturated monomer can also be a functional monomer including, for example, monomers containing hydroxy, amido, aldehyde, ureido, polyether, glycidylalkyl, keto functional groups or combinations thereof. These functional monomers are generally present in the macromonomer at a level of from 0.5 weight percent to 15 weight percent and more preferably from 1 weight percent to 3 weight percent, based on the total weight of the graft copolymer. Examples of functional monomers include ketofunctional monomers such as the acetoacetoxy esters of hydroxyalkyl acrylates and methacrylates (e.g., acetoacetoxyethyl methacrylate) and keto-containing amides (e.g., diacetone acrylamide); allyl alkyl methacrylates or acrylates; glycidylalkyl methacrylates or acrylates; or combinations thereof. Such functional monomers can provide crosslinking, if desired.
The macromonomer also preferably contains as polymerized units 10 weight percent or less, preferably 5 weight percent or less, more preferably 2 weight percent or less and most preferably 1 weight percent or less acid containing monomer, based on the total weight of the macromonomer. In a particularly preferred embodiment, the macromonomer contains no acid containing monomer. The term xe2x80x9cacid containing monomerxe2x80x9d, as used herein, refers to any ethylenically unsaturated monomer that contains one or more acid functional groups or functional groups that are capable of forming an acid (e.g., an anhydride such as methacrylic anhydride or tertiary butyl methacrylate). Examples of acid containing monomers include, for example, carboxylic acid bearing ethylenically unsaturated monomers such as acrylic acid, methacrylic acid, itaconic acid, maleic acid and fumaric acid; acryloxypropionic acid and (meth)acryloxypropionic acid; sulphonic acid-bearing monomers, such as styrene sulfonic acid, sodium vinyl sulfonate, sulfoethyl acrylate, sulfoethyl methacrylate, ethylmethacrylate-2-sulphonic acid, or 2-acrylamido-2-methylpropane sulphonic acid; phosphoethylmethacrylate; the corresponding salts of the acid containing monomer; or combinations thereof.
As polymerized, the macromonomer is substantially (including completely) free of mercapto-olefin compounds. The term xe2x80x9csubstantially freexe2x80x9d, as used herein, means that the macromonomer contains, as polymerized, 5 mole percent or less mercapto-olefin compounds, based on the total weight of the macromonomer. Preferably, the macromomer contains 2 mole percent or less mercapto-olefin compounds, with 1 mole percent or less being more preferred. Even more preferably, the macromonomer contains 0.5 mole or less percent mercapto-olefin compounds. In certain particularly preferred embodiments, the macromonomer contains completely no (i.e., 0 mole percent) mercapto-olefin compounds. Some examples of suitable mercapto-olefin compounds are those as described in U.S. Pat. No. 5,247,000 to Amick. The mercapto-olefin compounds described in Amick have ester functional groups, which are susceptible to hydrolysis.
In a preferred embodiment of the present invention, the macromonomer is composed of at least 20 weight percent, more preferably from 50 weight percent to 100 weight percent, and most preferably from 80 to 100 weight percent of at least one xcex1-methyl vinyl monomer, a non xcex1-methyl vinyl monomer terminated with a xcex1-methyl vinyl monomer, or combinations thereof. In a particularly preferred embodiment of the present invention, the macromonomer contains as polymerized units from 90 weight percent to 100 weight percent xcex1-methyl vinyl monomers, non xcex1-methyl vinyl monomers terminated with xcex1-methyl vinyl monomers, or combinations thereof, based on the total weight of the macromonomer. Suitable xcex1-methyl vinyl monomers include, for example, methacrylate esters, such as C1 to C18 normal or branched alkyl esters of methacrylic acid, including methyl methacrylate, ethyl methacrylate, butyl methacrylate, 2-ethylhexyl methacrylate, isobornyl methacrylate, lauryl methacrylate, or stearyl methacrylate; hydroxyalkyl methacrylates such as hydroxyethyl methacrylate; glycidylmethacrylate; phenyl methacrylate; methacrylamide; methacrylonitrile; or combinations thereof. An example of a non xcex1-methyl vinyl monomer terminated with an xcex1-methyl vinyl monomer includes styrene terminated by xcex1-methyl styrene.
The macromonomers employed in the present invention may be prepared by a variety of methods which would be readily apparent to one skilled in the art once armed with the teachings of the present disclosure. For example, the macromonomer may be prepared by a high temperature (e.g., at least about 150xc2x0 C.) continuous process such as disclosed in U.S. Pat. No. 5,710,227 or EP-A-1,010,706, published Jun. 21, 2000. In a preferred continuous process, a reaction mixture of ethylenically unsaturated monomers is passed through a heated zone having a temperature of at least 150xc2x0 C., and more preferably at least 275xc2x0 C. The heated zone may also be maintained at a pressure above atmospheric pressure (e.g., greater than about 30 bar). The reaction mixture of monomers may also optionally contain a solvent such as water, acetone, methanol, isopropanol, propionic acid, acetic acid, dimethylformamide, dimethylsulfoxide, methylethylketone, or combinations thereof which are stripped out of the polymer after polymerization.
The macromonomer useful in the present invention may also be prepared by polymerizing ethylenically unsaturated monomers in the presence of a free radical initiator and a catalytic metal chelate chain transfer agent (e.g., a transition metal chelate). Such a polymerization may be carried out by a solution, bulk, suspension, or emulsion polymerization process. Suitable methods for preparing the macromonomer using a catalytic metal chelate chain transfer agent are disclosed in for example U.S. Pat. Nos. 4,526,945, 4,680,354, 4,886,861, 5,028,677, 5,362,826, 5,721,330, and 5,756,605; European publications EP-A-0199,436, and EP-A-0196783; and PCT publications WO 87/03605, WO 96/15158, and WO 97/34934.
In accordance with preferred embodiments of the present invention, the macromonomer may be prepared by an aqueous emulsion free radical polymerization process. It has been surprisingly and unexpectedly found that this polymerization process may be advantageously carried out without the use of prior art chain transfer agents such as, for example, mercaptans, hypophosphites, sulfates, and alcohols. Such prior art chain transfer agents may be undesirable in that they may exhibit offensive odors that may be imparted to the polymer, and may also add to the cost of the process, impart undesired functionality to the polymer, introduce undesired salts into the process, and introduce additional process steps, including product separation.
The aqueous emulsion free radical polymerization process is preferably conducted using a transition metal chelate complex as a chain transfer agent. Preferably, the transition metal chelate complex is a cobalt (II) or (III) chelate complex such as, for example, dioxime complexes of cobalt, cobalt II porphyrin complexes, or cobalt II chelates of vicinal iminohydroxyimino compounds, dihydroxyimino compounds, diazadihydroxyiminodialkyldecadienes, or diazadihydroxyiminodialkylundecadienes, or combinations thereof. These complexes may optionally include bridging groups such as BF2, and may also be optionally coordinated with ligands such as water, alcohols, ketones, and nitrogen bases such as pyridine. Additional suitable transition metal complexes are disclosed, for example, in U.S. Pat. Nos. 4,694,054; 5,770,665; 5,962,609; and 5,602,220. A preferred cobalt chelate complex useful in the preparation of the macromonomers of the present invention is Co II (2,3-dioxyiminobutane-BF2)2, the Co III analogue of the aforementioned compound, or combinations thereof. The spatial arrangements of such complexes are disclosed, for example, in EP-A-199436 and U.S. Pat. No. 5,756,605.
In preparing macromonomer by an aqueous emulsion polymerization process using a transition metal chelate chain transfer agent, at least one ethylenically unsaturated monomer may be polymerized in the presence of a free radical initiator and the transition metal chelate according to conventional aqueous emulsion polymerization techniques. Preferably, the ethylenically unsaturated monomer is an xcex1-methyl vinyl monomer as previously described herein.
The polymerization reaction to form the macromonomer is preferably conducted at a temperature of from 20xc2x0 C. to 150xc2x0 C., and more preferably from 40xc2x0 C. to 95xc2x0 C. The solids level at the completion of the polymerization is typically from 5 weight percent to 65 weight percent, and more preferably from 30 weight percent to 50 weight percent, based on the total weight of the aqueous emulsion.
The concentration of initiator and transition metal chelate chain transfer agent used during the polymerization process is preferably chosen to obtain the desired degree of polymerization of the macromonomer. Preferably, the concentration of initiator is from 0.2 weight percent to 3 weight percent, and more preferably from 0.5 weight percent to 1.5 weight percent, based on the total weight of monomer. Preferably, the concentration of transition metal chelate chain transfer agent is from 5 ppm to 200 ppm, and more preferably from 10 ppm to 100 ppm, based on the total moles of monomer used to form the macromonomer.
The ethylenically unsaturated monomer, initiator, and transition metal chelate chain transfer agent may be added in any manner known to those skilled in the art to carry out the polymerization. For example, the monomer, initiator and transition metal chelate may all be present in the aqueous emulsion at the start of the polymerization process (i.e., a batch process). Alternatively, one or more of the components may be gradually fed to an aqueous solution (i.e., a continuous or semi-batch process). For example, it may be desired to gradually feed the entire or a portion of the initiator, monomer, and/or transition metal chelate to a solution containing water and surfactant. In a preferred embodiment, at least a portion of the monomer and transition metal chelate are gradually fed during the polymerization, with the remainder of the monomer and transition metal chelate being present in the aqueous emulsion at the start of the polymerization. In this embodiment, the monomer may be fed as is, or suspended or emulsified in an aqueous solution prior to being fed.
Any suitable free radical initiator may be used to prepare the macromonomer. The initiator is preferably selected based on such parameters as its solubility in one or more of the other components (e.g., monomers, water); half life at the desired polymerization temperature (preferably a half life within the range of from 30 minutes to 10 hours), and stability in the presence of the transition metal chelate. Suitable initiators include for example azo compounds such as 2,2xe2x80x2-azobis (isobutyronitrile), 4,4xe2x80x2-azobis(4-cyanovaleric acid), 2,2xe2x80x2-azobis [2-methyl-N-(1,1-bis(hydroxymethyl)-2-(hydroxyethyl)]-propionamide, and 2,2xe2x80x2-azobis [2-methyl-N-(2-hydroxyethyl)]-propionamide; peroxides such as t-butyl hydroperoxide, benzoyl peroxide; sodium, potassium, or ammonium persulphate or combinations thereof. Redox initiator systems may also be used, such as for example persulphate or peroxide in combination with a reducing agent such as sodium metabisulphite, sodium bisulfite, sodium formaldehyde sulfoxylate, isoascorbic acid, or combinations thereof. Metal promoters, such as iron, may also optionally be used in such redox initiator systems. Also, buffers, such as sodium bicarbonate may be used as part of the initiator system.
An emulsifier is also preferably present during the aqueous emulsion polymerization process to prepare the macromonomer. Any emulsifier may be used that is effective in emulsifying the monomers such as for example anionic, cationic, or nonionic emulsifiers. In a preferred embodiment, the emulsifier is anionic such as for example sodium, potassium, or ammonium salts of dialkylsulphosuccinates; sodium, potassium, or ammonium salts of sulphated oils; sodium, potassium, or ammonium salts of alkyl sulphonic acids, such as sodium dodecyl benzene sulfonate; sodium, potassium, or ammonium salts of alkyl sulphates, such as sodium lauryl sulfate; ethoxylated alkyl ether sulfates; alkali metal salts of sulphonic acids; C12 to C24 fatty alcohols, ethoxylated fatty acids or fatty amides; sodium, potassium, or ammonium salts of fatty acids, such as Na stearate and Na oleate; or combinations thereof. The amount of emulsifier in the aqueous emulsion is preferably from 0.05 weight percent to 10 weight percent, and more preferably from 0.3 weight percent to 3 weight percent, based on the total weight of the monomers.
The macromonomer aqueous emulsion may be formed in any manner known to those skilled in the art. For example, the macromonomer, produced by any known method, may be isolated as a solid (e.g., spray dried) and emulsified in water. Also, for example, the macromonomer, if prepared via an emulsion or aqueous based polymerization process, may be used as is, or diluted with water or concentrated to a desired solids level.
In a preferred embodiment of the present invention, the macromonomer aqueous emulsion is formed from the emulsion polymerization of an ethylenically unsaturated monomer in the presence of a transition metal chelate chain transfer agent as described previously herein. This embodiment is preferred for numerous reasons. For example, the macromonomer polymerization can be readily controlled to produce a desired particle size distribution (preferably narrow, e.g., polydispersity less than 2). Also, for example, additional processing steps, such as isolating the macromonomer as a solid, can be avoided, leading to better process economics. In addition, the macromonomer, macromonomer aqueous emulsion and the graft copolymer can be prepared by consecutive steps in a single reactor which is desirable in a commercial manufacturing facility.
The macromonomer aqueous emulsion useful in the present invention contains from 20 weight percent to 60 weight percent, and more preferably from 30 weight percent to 50 weight percent of at least one water insoluble macromonomer, based on the total weight of macromonomer aqueous emulsion. The macromonomer aqueous emulsion may also contain mixtures of macromonomer. Preferably, the macromonomer aqueous emulsion contains less than 5 weight percent and more preferably less than 1 weight percent of residual ethylenically unsaturated monomer, based on the total weight of macromonomer aqueous emulsion.
The water insoluble macromonomer particles preferably have a particle size to permit the formation of a graft copolymer of a desired particle size. Preferably, the macromonomer particles have a weight average particle size of from 50 nm to 600 nm, and more preferably from 80 nm to 200 nm, as measured by Capillary Hydrodynamic Fractionation technique using a Matec CHDF 2000 particle size analyzer equipped with a HPLC type Ultra-violet detector.
The macromonomer aqueous emulsion may also include one or more emulsifying agents. The type and amount of emulsifying agent is preferably selected in a manner to produce the desired particle size. Suitable emulsifying agents include those previously disclosed for use in preparing the macromonomer by an emulsion polymerization process. Preferred emulsifying agents are anionic surfactants such as, for example, sodium lauryl sulfate, sodium dodecylbenzene sulfonate, sulfated and ethoxylated derivatives of nonylphenols and fatty alcohols. The total level of emulsifying agent, based on the total weight of macromonomer is preferably from 0.2 weight percent to 5 weight percent and more preferably from 0.5 weight percent to 2 weight percent.
The macromonomer thus prepared is preferably emulsion polymerized with an ethylenically unsaturated monomer to form a graft copolymer composition. The polymerization is preferably carried out by providing the macromonomer as water insoluble particles in an aqueous emulsion and the ethylenically unsaturated monomer in a monomer composition. In certain preferred embodiments, at least a portion of the macromonomer aqueous emulsion is combined with at least a portion of the monomer composition to form a polymerization reaction mixture that is then polymerized in the presence of an initiator.
Although in no way intending to be bound in theory, it is believed that by providing the macromonomer in the form of water insoluble macromonomer particles in an aqueous emulsion, and the ethylenically unsaturated monomer in a separate monomer composition, upon combination, the ethylenically unsaturated monomer diffuses into the macromonomer particles where the polymerization occurs. Preferably, the diffusion of the ethylenically unsaturated monomer into the macromonomer particles is evidenced by swelling of the macromonomer particles.
The monomer composition useful in the present invention preferably contains at least one kind of ethylenically unsaturated monomer. The monomer composition may contain all (i.e., 100%) monomer, or may contain monomer dissolved or dispersed in an organic solvent and/or water. Preferably, the level of monomer in the monomer composition is from 50 weight percent to 100 weight percent, more preferably from 60 weight percent to 90 weight percent, and most preferably from 70 weight percent to 80 weight percent, based on the total weight of the monomer composition. Examples of organic solvents that may be present in the monomer composition include C6 to C14 alkanes, such as, for example, hexane, heptane, octane, nonane, decane, undecane, dodecane, tridecane, and tetradecane. The organic solvent in the monomer composition will preferably be no more than 30 weight percent, and more preferably no more than 5 weight percent, based on the total weight of the monomer composition and is stripped out of the polymer after polymerization.
In addition to water and/or organic solvent, the monomer composition may also optionally contain monomers containing functional groups, such as, for example, monomers containing hydroxy, amido, aldehyde, ureido, polyether, glycidylalkyl, keto groups or combinations thereof. These other monomers are generally present in the monomer composition at a level of from 0.5 weight percent to 15 weight percent, and more preferably from 1 weight percent to 3 weight percent based on the total weight of the graft copolymer. Examples of functional monomers include ketofunctional monomers such as the acetoacetoxy esters of hydroxyalkyl acrylates and methacrylates (e.g., acetoacetoxyethyl methacrylate) and keto-containing amides (e.g., diacetone acrylamide); allyl alkyl methacrylates or acrylates; glycidylalkyl methacrylates or acrylates; or combinations thereof. Such functional monomer can provide crosslinking if desired.
In a preferred embodiment, the monomers in the monomer composition may be pre-emulsified in water to form a monomer aqueous emulsion. Preferably, the monomer aqueous emulsion may contain monomer droplets having a droplet size from 1 micron to 100 microns, and more preferably from 5 micron to 50 microns. Any suitable emulsifying agent may be used, such as those previously described, to emulsify the monomer to the desired monomer droplet size. Preferably, the level of emulsifying agent, if present, may be from 0.2 weight percent to 2 weight percent based on the total weight of monomer in the monomer composition.
The ethylenically unsaturated monomer of the monomer composition is preferably selected to provide the desired properties in the resulting copolymer composition. Suitable ethylenically unsaturated monomers include for example methacrylate esters, such as C1 to C18 normal or branched alkyl esters of methacrylic acid, including methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, stearyl methacrylate, isobornyl methacrylate; acrylate esters, such as C1 to C18 normal or branched alkyl esters of acrylic acid, including methyl acrylate, ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate; styrene; substituted styrenes, such as methyl styrene, xcex1-methyl styrene or t-butyl styrene; olefinically unsaturated nitriles, such as acrylonitrile or methacrylonitrile; olefinically unsaturated halides, such as vinyl chloride, vinylidene chloride or vinyl fluoride; vinyl esters of organic acids, such as vinyl acetate; N-vinyl compounds such as N-vinyl pyrrolidone; acrylamide; methacrylamide; substituted acrylamides; substituted methacrylamides; hydroxyalkylmethacrylates such as hydroxyethylmethacrylate; hydroxyalkylacrylates; dienes such as 1,3-butadiene and isoprene; vinyl ethers; or combinations thereof. The ethylenically unsaturated monomer can also be an acid containing monomer or a functional monomer, such as those previously described herein. Preferably, the ethylenically unsaturated monomer of the monomer composition does not contain amino groups.
In a preferred embodiment, the monomer composition may include one or more ethylenically unsaturated monomers selected from C1 to C18 normal or branched alkyl esters of acrylic acid, including methyl acrylate, ethyl acrylate, n-butyl acrylate and 2-ethylhexyl acrylate; styrene; substituted styrenes, such as methyl styrene, xcex1-methyl styrene or t-butyl styrene; butadiene or combinations thereof.
As previously mentioned, the macromonomer aqueous emulsion and monomer composition are preferably combined to form a polymerization reaction mixture, and then polymerized in the presence of a free radical initiator to form an aqueous copolymer composition. The term xe2x80x9cpolymerization reaction mixture,xe2x80x9d as used herein, refers to the resulting mixture formed when at least a portion of the macromonomer aqueous emulsion and at least a portion of the monomer composition are combined. The polymerization reaction mixture may also contain initiator or any other additive used during the polymerization. Thus, the polymerization reaction mixture is a mixture that changes in composition as the macromonomer and monomer in the monomer composition are reacted to form graft copolymer.
The macromonomer aqueous emulsion and monomer composition may be combined in various ways to carry out the polymerization. For example, the macromonomer aqueous emulsion and the monomer composition may be combined prior to the start of the polymerization reaction to form the polymerization reaction mixture. Alternatively, the monomer composition could be gradually fed into the macromonomer aqueous emulsion, or the macromonomer aqueous emulsion could be gradually fed into the monomer composition. It is also possible that only a portion of the macromonomer aqueous emulsion and/or monomer composition be combined prior to the start of the polymerization with the remaining monomer composition and/or macromonomer aqueous emulsion being fed during the polymerization.
The initiator may also be added in various ways. For example, the initiator may be added in xe2x80x9cone shotxe2x80x9d to the macromonomer aqueous emulsion, the monomer composition, or a mixture of the macromonomer aqueous emulsion and the monomer composition at the start of the polymerization. Alternatively, all or a portion of the initiator can be co-fed as a separate feed stream, as part of the macromonomer aqueous emulsion, as part of the monomer composition, or any combination of these methods.
The preferred method of combining the macromonomer aqueous emulsion, the monomer composition, and initiator may depend on such factors as the desired graft copolymer composition, and ultimately, the desired PSA properties of the adhesive compositions. For example, the distribution of the macromonomer as a graft along the backbone may be affected by the concentrations of both the macromonomer and the ethylenically unsaturated monomers at the time of the polymerization. In this regard, a batch process may afford high concentration of both the macromonomer and the ethylenically unsaturated monomers at the onset of the polymerization whereas a semi-continuous process will typically keep the ethylenically unsaturated monomer concentration low during the polymerization. Thus, through the method in which the macromonomer aqueous emulsion and monomer composition are combined, it may be possible to control, for example, the number of macromonomer grafts per polymer chain, the distribution of graft in each chain, and/or the length of the polymer backbone.
Initiators, useful in polymerizing the macromonomer and ethylenically unsaturated monomer to form the graft copolymer of the present invention, may include any suitable initiator for emulsion polymerizations known to those skilled in the art. The selection of the initiator will depend on such factors as the initiator""s solubility in one or more of the reaction components (e.g. monomer, macromonomer, water); and half-life at the desired polymerization temperature (preferably a half-life within the range of from 30 minutes to 10 hours). Suitable initiators include those previously described herein in connection with forming the macromonomer, such as azo compounds such as 4,4xe2x80x2-azobis(4-cyanovaleric acid), peroxides such as t-butyl hydroperoxide; sodium, potassium, or ammonium persulfate; redox initiator systems such as, for example, persulphate or peroxide in combination with a reducing agent such as sodium metabisulfite, sodium bisulfite, sodium formaldehyde sulfoxylate, isoascorbic acid; or combinations thereof. Metal promoters, such as iron; and buffers, such as sodium bicarbonate, may also be used in combination with the initiator. Additionally, Controlled Free Radical Polymerization (CFRP) methods such as Atom Transfer Radical Polymerization; or Nitroxide Mediated Radical Polymerization may be used. Preferred initiators include azo compounds such as 4,4xe2x80x2-azobis(4-cyanovaleric acid).
The amount of initiator used will depend on such factors as the copolymer desired and the initiator selected. Preferably, from 0.1 weight percent to 1 weight percent initiator is used, based on the total weight of monomer and macromonomer.
The polymerization temperature may depend on the type of initiator chosen and the desired polymerization rates. Preferably, however, the macromonomer and ethylenically unsaturated monomer are polymerized at a temperature of from room temperature to 150xc2x0 C., and more preferably from 40xc2x0 C. to 95xc2x0 C.
The amount of macromonomer aqueous emulsion and monomer composition added to form the polymerization reaction mixture will depend on such factors, for example, as the concentrations of macromonomer and ethylenically unsaturated monomer in the macromonomer aqueous emulsion and monomer composition, respectively, and the desired copolymer composition. Preferably, the macromonomer aqueous emulsion and monomer composition are added in amounts to provide a copolymer containing as polymerized units of from 1 weight percent to 30 weight percent, more preferably from 2 weight percent to 15 weight percent, and most preferably from 2.5 weight percent to 10 weight percent macromonomer, and from 70 weight percent to 99 weight percent, more preferably from 85 weight percent to 98 weight percent, and most preferably from 90 weight percent to 97.5 weight percent ethylenically unsaturated monomer.
It would be readily apparent to one skilled in the art that other components used in conventional emulsion polymerizations may optionally be used with the methods of the present invention once armed with the teachings of this disclosure. For example, to reduce the molecular weight of the resulting graft copolymer, the polymerization may optionally be conducted in the presence of one or more chain transfer agents, such as n-dodecyl mercaptan, thiophenol; halogen compounds such as bromotrichloromethane; or combinations thereof. Also, additional initiator and/or catalyst may be added to the polymerization reaction mixture at the completion of the polymerization reaction to reduce any residual monomer, (e.g., chasing agents). Suitable initiators or catalysts include those initiators previously described herein. In addition, the chain transfer capacity of a macromonomer through addition-fragmentation can be utilized in part to reduce molecular weight through appropriate design of monomer compositions and polymerization conditions. See e.g., E. Rizzardo, et. al., Prog. Pacific Polym. Sci., 1991, 1, 77-88; G. Moad, et. al., WO 96/15157.
The resulting aqueous copolymer composition formed by polymerization of the macromonomer and the ethylenically unsaturated monomer in the monomer composition preferably has a solids level of from 30 weight percent to 65 weight percent and more preferably from 40 weight percent to 60 weight percent. In addition, the aqueous copolymer composition preferably contains copolymer particles that are water insoluble and have a particle size of from 60 nm to 600 nm, and more preferably from 80 nm to 200 nm. The copolymer compositions are suitable for incorporating into the aqueous emulsion-based, PSA adhesive compositions of the present invention by itself or with other additives.
In certain preferred embodiments, the graft copolymer formed has a backbone containing, as polymerized units, the ethylenically unsaturated monomer from the monomer composition, and one or more side chains, pendent from the backbone, containing the macromonomer. Preferably, each side chain is formed from one macromonomer grafted to the backbone. The number average molecular weight of the macromonomer side chains is preferably in the range of from 2,000 to 50,000 g/mole, and more preferably in the range of from 4,000 to 35,000 g/mole. The total weight average molecular weight of the graft copolymer is preferably in the range of from 50,000 to 2,000,000, and more preferably from 100,000 to 1,000,000. Weight average molecular weights as used herein can be determined by size exclusion chromatography.
The copolymer particles of the aqueous copolymer composition can be isolated, for example, by spray drying or coagulation. However, it is preferable to use the copolymer aqueous composition as is, i.e., without further processing.
In a preferred embodiment of the present invention, the polymerization is conducted in two stages. In the first stage, the macromonomer is formed in an aqueous emulsion polymerization process, and in the second stage the macromonomer is polymerized with the ethylenically unsaturated monomer in an emulsion. For efficiency, preferably these two stages are conducted in a single vessel. For example, in the first stage, the macromonomer aqueous emulsion may be formed by polymerizing in an aqueous emulsion at least one first ethylenically unsaturated monomer to form water insoluble macromonomer particles. This first stage polymerization is preferably conducted using a transition metal chelate chain transfer agent as previously described herein. After forming the macromonomer aqueous emulsion, a second emulsion polymerization is then preferably performed in the same vessel to polymerize any unreacted first ethylenically unsaturated macromonomer with at least one second ethylenically unsaturated monomer. This second stage may be conducted for example by directly adding (e.g., all at once or by a gradual feed) the monomer composition and initiator to the macromonomer aqueous emulsion. One main advantage of this embodiment is that the macromonomer does not have to be isolated, and the second polymerization can take place simply by adding the monomer composition and initiator to the macromonomer aqueous emulsion.
In another preferred embodiment of the present invention, the polymerization of the macromonomer and ethylenically unsaturated monomer is at least partially performed in the presence of an acid containing monomer, acid containing macromonomer, or combinations thereof. The acid containing monomer or acid containing macromonomer may be added in any manner to the polymerization reaction mixture. Preferably, the acid containing monomer or acid containing macromonomer is present in the monomer composition. The acid containing monomer or acid containing macromonomer may also be added as a separate stream to the polymerization reaction mixture.
The amount of acid containing monomer or acid containing macromonomer added to the polymerization reaction mixture is preferably from 0.2 weight percent to 10 weight percent, more preferably from 0.5 weight percent to 5 weight percent, and most preferably from 1 weight percent to 2 weight percent, based on the total weight of monomer and macromonomer added to the polymerization reaction mixture.
Acid containing monomers which may be used in this embodiment may include ethylenically unsaturated monomers bearing acid functional or acid forming groups such as those previously described herein.
The acid containing macromonomer useful in this embodiment is any low molecular weight polymer having at least one terminal ethylenically unsaturated group that is capable of being polymerized in a free radical polymerization process, and that is formed from at least one kind of acid containing monomer. Preferably, the amount of acid containing monomer in the acid containing macromonomer is from 50 weight percent to 100 weight percent, more preferably from 90 weight percent to 100 weight percent, and most preferably from about 95 weight percent to 100 weight percent.
The acid containing macromonomer may be prepared according to any technique known to those skilled in the art such as those previously described herein. In a preferred embodiment of the present invention, the acid containing macromonomer is prepared by a solution polymerization process using a free radical initiator and transition metal chelate complex. An example of such a process is disclosed in U.S. Pat. No. 5,721,330. Preferred acid containing monomers used to form the acid containing macromonomer are xcex1-methyl vinyl monomers such as methacrylic acid.
Although in no way intending to be bound by theory, it is believed that the acid containing macromonomer is attached to the surface of the water insoluble graft copolymer particles and provides stability. By xe2x80x9cattached,xe2x80x9d as used herein, it is believed that the acid containing macromonomer is bound in some manner (e.g., covalent, hydrogen bonding, ionic) to a polymer chain in the particle. Preferably, the acid containing macromonomer is covalently bound to a polymer chain in the particle. It has been found that the acid containing macromonomer provides stability to the particles such that the aqueous copolymer composition produced exhibits unexpected improved shear stability; freeze thaw stability; and stability to additives in formulations, as well as reduction of coagulums during the polymerization. Although improved stability can be achieved using acid containing monomer, these benefits are most dramatic when an acid containing macromonomer is used.
In another preferred embodiment of the present invention, a macromolecular organic compound having a hydrophobic cavity is present in the polymerization medium used to form the macromonomer and/or aqueous copolymer composition. Preferably, the macromolecular organic compound is used when copolymerizing ethylenically unsaturated monomers with very low water solubility such as lauryl or stearyl acrylates and/or methacrylates. By xe2x80x9clow water solubilityxe2x80x9d it is meant a water solubility at 25xc2x0 C. to 50xc2x0 C. of no greater than 50 millimoles/liter. For example, the macromolecular organic compound may be added to the monomer composition, the macromonomer aqueous emulsion, or the polymerization reaction mixture used to form the aqueous copolymer composition. Also, for example the macromolecular organic compound may be added to an aqueous emulsion of ethylenically unsaturated monomer used to form the macromonomer. Suitable techniques for using a macromolecular organic compound having a hydrophobic cavity are disclosed in, for example, U.S. Pat. No. 5,521,266.
Preferably, the macromolecular organic compound having a hydrophobic cavity is added to the polymerization reaction mixture to provide a molar ratio of macromolecular organic compound to low water solubility monomer or macromonomer of from 5:1 to 1:5000 and more preferably from 1:1 to 1:500.
Macromolecular organic compounds having a hydrophobic cavity useful in the present invention include for example cyclodextrin or cyclodextrin derivatives; cyclic oligosaccharides having a hydrophobic cavity such as cycloinulohexose, cycloinuloheptose, or cycloinuloctose; calyxarenes; cavitands; or combinations thereof. Preferably, the macromolecular organic compound is xcex2-cyclodextrin, more preferably methyl-xcex2-cyclodextrin.
Monomers having low water solubility include for example primary alkenes; styrene and alkylsubstituted styrene; xcex1-methyl styrene; vinyltoluene; vinyl esters of C4 to C30 carboxylic acids, such as vinyl 2-ethylhexanoate, vinyl neodecanoate; vinyl chloride; vinylidene chloride; N-alkyl substituted (meth)acrylamide such as octyl acrylamide and maleic acid amide; vinyl alkyl or aryl ethers with (C3-C30) alkyl groups such as stearyl vinyl ether; (C1-C30) alkyl esters of (meth)acrylic acid, such as methyl methacrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, benzyl (meth)acrylate, lauryl (meth)acrylate, oleyl (meth)acrylate, palmityl (meth)acrylate, stearyl (meth)acrylate; unsaturated vinyl esters of (meth)acrylic acid such as those derived from fatty acids and fatty alcohols; multifunctional monomers such as pentaerythritol triacrylate; monomers derived from cholesterol or combinations thereof.
The aqueous copolymer composition, in addition to the copolymer particles, preferably contains less than 10 weight percent, and more preferably less than 1 weight percent of organic solvent. In a most preferred embodiment, the aqueous copolymer composition contains no organic solvent.
The adhesive compositions of the present invention may include from 30 weight percent to 70 weight percent solids of the grafted copolymer of the present invention dispersed in an aqueous medium. The adhesive compositions of the present invention may, optionally, further include from 0.1 to 60 weight percent solids of the grafted copolymer of at least one additive such as, but not limited to, tackifiers, emulsifiers, pigments, fillers, curing agents, thickeners, wetting agents, defoamers, biocides, adhesion promoters, humectants, colorants, waxes, UV stabilizers, antioxidants, and the like.
In preferred embodiments, tackifying resins may be added to the adhesive composition to increase tack and peel adhesion. However, the tackifying resin selected should preferably be compatible with the polymeric backbone, rather than the grafted polymer or macromonomer, to maintain high shear properties. Higher amounts of tackifying resins, or tackifiers, may have a negative effect on shear properties.
Additives, if used, can be added to the adhesive compositions of the present invention by any suitable technique, such as by mixing or blending, to uniformly incorporate the additive into the adhesive compositions. The additive is preferably added to the adhesive composition in the form of a liquid, an aqueous emulsion, or an emulsified solution. In more preferred embodiments, the adhesive composition of the present invention is prepared by adding an aqueous emulsion containing the graft copolymer particles dispersed therein and, optionally, any other additives, of the adhesive composition to an aqueous emulsion and agitating the combination to form a substantially uniform emulsion.
In the PSA compositions of the present invention, it is preferred that the percentage of grafting of the macromomoner side chains to the monomeric backbone ranges from 50% to 100% (i.e., all of the macromonomer in the system is grafted). Preferably, the percentage of grafting of the macromonomer to the monomeric backbone is 60% or greater, more preferably 70% or greater, and even more preferably 90% or greater. The term xe2x80x9cpercentage of graftingxe2x80x9d, as used herein, means the amount of macromonomer copolymerized onto the backbone divided by the total amount of charged macromonomer into the reaction times 100. The percentage of grafting is measured via HPLC to measure the amount of unreacted macromonomer.
It is generally believed that the Tg of the hard phase strongly influences the shear properties of the resulting adhesive compositions. The degree of compatibility of the respective Tg values for the hard and soft phases also effects the shear and other properties of the adhesive compositions. To achieve the necessary balance of adhesive properties, the hard phase of the graft copolymer particles dispersed within the adhesive compositions of the present invention preferably has a midpoint Tg value of 40xc2x0 C. or greater. More preferably, the midpoint Tg value of the hard phase of the graft polymer is 70xc2x0 C. or greater. Even more preferably, the midpoint Tg value of the hard phase of the graft polymer is 90xc2x0 C. or greater. Although the Tg value of the soft phase is not as strong an influence on shear properties as the hard phase, it is preferred that the midpoint Tg value of the soft phase is xe2x88x9220xc2x0 C. The Tg values set forth herein are based on measured values obtained, for example, by differential scanning calorimetry of the respective polymer.
In certain preferred embodiments, the PSA properties of the adhesive compositions of the present invention may be improved by controlling the number average molecular weight of the one or more macromonomers grafted to the polymeric backbone within the graft copolymer particles. It is generally believed that the desired balance of PSA properties is obtained when the molecular weight of the high Tg graft is sufficiently high so as to cause phase separation of the high Tg graft. Preferably, the number average molecular weight of the grafted macromonomer ranges from 2,000 to 50,000, and more preferably from 4,000 to 35,000.
The amount of grafted macromonomer or macromonomers, by weight percentage of the copolymer composition, within the PSA compositions of the present invention may have an influence on the resultant PSA properties of the adhesive. For example, if the amount of grafted macromonomer is below a certain value, the grafted polymer material may not provide sufficient reinforcement so as to improve shear properties. However, if the amount of grafted macromomoner is too high, the tack properties of the PSA may be reduced. Preferably, the amount, by weight percentage of the copolymer composition, of grafted macromonomer should range from 1 to 30%, more preferably from 2 to 15%, and even more preferably from 2.5 to 10% to improve shear while maintaining tack.
An adhesive article, particularly a PSA article, may be made by applying a coating of the adhesive composition of the present invention to a primary substrate and allowing the coating to dry, thereby providing an adhesive layer consisting of the solids portion of the adhesive composition covering a portion of the surface of the substrate.
The coating of adhesive composition can generally be applied to at least a portion of at least one surface of the primary substrate by any convenient method such as, for example, roll coating, wire-wound rod coating, knife coating, or curtain coating, and allowed to dry to form a dry adhesive layer on the coated portion of the surface of substrate. The adhesive composition may also be applied as a continuous coating or a discontinuous coating on the surface of the primary substrate.
In one embodiment, the adhesive or PSA composition may be applied to a surface of the primary substrate in an amount effective to provide a dry adhesive layer 5 grams per square meter (g/m2) to 100 g/m2 on the coated portion of the surface of the primary substrate.
In a further embodiment, the primary substrate may be a flexible, sheet-like material such as, for example, a sheet of paper, a polymer film, a textile fabric or a nonwoven fiber sheet, and the adhesive article of the present invention is correspondingly a sheet-like material such as, for example, a pressure sensitive adhesive tape, a pressure sensitive adhesive label or a pressure sensitive adhesive film.
In preferred from, the adhesive article is a PSA tape having an adhesive coated surface and an opposite non-coated surface. In a preferred embodiment, the article may include a release layer or coating, e.g., a polymer film, for temporarily covering the adhesive layer prior to use. In an alternative preferred embodiment, wherein an adhesive tape is provided in the form of a concentrically wound roll, the non-coated surface of the underlying layer of tape functions as a release layer for the adhesive layer. In yet a further preferred embodiment, the PSA article may be comprised of a face material, a layer of adhesive, a release coating, and a removable backing or liner.
The PSA composition may be applied to more than one surface of the primary substrate, for example, both sides of a strip of a polymer film may be coated to make a xe2x80x9cdouble-sidedxe2x80x9d adhesive tape.
The PSA articles of the present invention can be used by removing the release layer, if present, from the article and then applying an adhesive coated surface of the adhesive article to one or more secondary substrates or to one or more portions of a single secondary substrate to form a composite article wherein the substrates or primary substrate and secondary substrate portions are bonded together by an interposed dry adhesive layer.
Preferred secondary substrates include, but are not limited to, sheet-like materials such as, for example, paper products such as papers and paperboards, cardboards, corrugated cardboards, wood, metal films, polymer films and composite substrates. The terminology xe2x80x9ccomposite substratesxe2x80x9d, as used herein, means substrates consisting of a combination of dissimilar substrate materials such as polymer-coated paperboards or cardboards such, for example, wax-coated cardboard, and bonded wood products such as, for example, particle boards.
The adhesive compositions prepared in accordance with the present invention are easily coated upon suitable flexible or inflexible backing materials by conventional coating techniques to produce coated adhesive sheet materials in accord with the present invention. The flexible backing material may be any material conventionally utilized as a tape backing or any other flexible material. Typical examples of flexible backing materials employed as conventional tape backings which may be useful for the adhesive compositions of the present invention include those made of paper, plastic films such as polypropylene, polyethylene, polyvinyl chloride, polyester (e.g., polyethylene terephthalate), cellulose acetate and ethyl cellulose.
Backings may also be prepared of fabric such as woven fabric formed of threads of synthetic or natural materials such as cotton, nylon, rayon, glass, ceramic material, and the like or nonwoven fabric such as air laid webs of natural or synthetic fibers or blends of these. The backing may also be formed of metal, metallized polymeric films, or ceramic sheet materials. The coated sheet materials may take the form of any article conventionally known to be utilized with adhesive or PSA compositions such as labels, tapes, signs, covers, marking indicia, and the like.
The PSA compositions of the present invention may also be suitable for use as removable adhesives. Ideal removable adhesives will not increase excessively in peel strength with time or exposure to heat and high humidity. Further, adhesive tapes made from such compositions should not whiten on exposure to high humidity or should not lift at the edges or form tunnels due to expansion of the tape backing when exposed to elevated temperatures. Certain PSA compositions of the present invention may exhibit no change in peel strength despite extended exposure to heat and humidity.
The PSA compositions of the present invention may also be suitable for high temperature adhesive systems. For example, certain PSA compositions of the present invention may exhibit high temperature shear resistance at temperatures approaching the Tg of the grafted macromonomer.